Stability of Molten-Phase Cs–V–O Catalysts for SO₃ Decomposition in Solar Thermochemical Water Splitting

Abstract

Supported molten cesium vanadate catalysts (Cs–V–O/SiO2) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO2) for SO3 decomposition at moderate temperatures (∼600 °C), which is essential as an O2 evolution reaction in solar thermochemical water splitting cycles. Stability testing of the catalyst over a 1000 h continuous reaction at 600 °C resulted in deactivation by ∼20% of the initial activity. Kinetic analysis of the activity versus time-on-stream indicated that the observed deactivation behavior can be divided into an induction period (≤100 h) and an acceleration period (>100 h). The deactivation is mainly caused by the vaporization loss of active components (Cs and V) from the molten phase. At the earliest stage, most vapor is generated in the upstream section of the catalyst bed and then redeposits therebelow. Upon repeating these vaporization and deposition cycles, Cs and V move gradually downstream. During this induction period, the deactivation is not obvious because the total Cs and V content of the catalyst bed remains almost unchanged. After this period, however, detachment of Cs and V from the downstream end of the catalyst bed induces accelerated deactivation. The vaporization loss was found to be significantly suppressed by inverting the catalyst bed every 100 h during the stability test. Consequently, this operation reduced the extent of catalyst deactivation from 20% to less than 10% of the initial activity.

Catalytic SO3 Decomposition Activity of SiO2-Supported Alkaline Earth Vanadates for Solar Thermochemical Water Splitting Cycles

“…15,16 Cs−V/SiO 2 exhibited high SO 3 conversion, which was comparable to that of Pt/TiO 2 at a moderate temperature of ∼600 °C. 33,34 Nevertheless, Cs−V is characterized by a low melting point (∼500 °C, Table 1). This limits the heating to temperatures above 700 °C to avoid rapid vaporization and loss of loaded Cs−V.…”

“…Their SO 3 decomposition activities were compared with those of previously reported alkaline and/or rare earth vanadates (A−V/SiO 2 and/or Re−V/SiO 2 ). 33,34,43 At a moderate temperature of 650 °C, the catalysts exhibited the following sequence of activity: A−V/SiO 2 > Ae−V/SiO 2 > Re−V/SiO 2 . At a higher temperature of 800 °C, however, Ae− V/SiO 2 was found to display the highest activity, which was superior to that of a supported Pt catalyst.…”

“…Another class of catalysts is metal oxides, − the activity of which at lower temperatures is considerably lower than that of Pt-based materials. We have previously investigated various metal vanadate-based catalyst systems, such as Cu–V, − K–V, and Cs–V, , supported on a porous SiO 2 support as potential alternatives to Pt catalysts. Unlike conventional solid oxide catalysts, metal vanadates can be regarded as molten phase catalysts because their melting points are nearly equal to or lower than the reaction temperature required for SO 3 decomposition.…”

Effects of coexisting oxoanions on SO₃ decomposition activity of molten-phase potassium metavanadate catalysts

“…7 may be associated with the formation of the solidstate phosphorous-rich phase under the present reaction condition. Importantly, the molten-state vanadate has more likely active than the solid phase, 11), 13) indicating that the low melting temperature of phosphate vanadate phases (KV 2 PO 8 ) is favorable for the formation of active molten phase.…”

Computational design of H 2 SO 4 decomposer combined with SOFC for thermochemical hydrogen production

“…), and intermetallic alloys (Ag-Pd) have been developed. [12][13][14][15][16] Sandia National Laboratories developed a lab-scale bayonet-type H 2 SO 4 decomposer (SAD), which contained three main parts: the boiler, superheater, and sulfur trioxide decomposer (STD), according to the operating temperature. 17 Nagarajan et al [17][18][19][20] proposed a numerical model based on computational fluid dynamics (CFD) to describe the fluid flow, species, and energy transfer in the SAD.…”